| Internet-Draft | ipcrypt | April 2025 |
| Denis | Expires 17 October 2025 | [Page] |
This document specifies methods for encrypting and obfuscating IP addresses, providing both deterministic format‑preserving and non‑deterministic constructions. These methods address privacy concerns raised in [RFC6973] and [RFC7258] regarding pervasive monitoring and data collection.¶
The methods apply uniformly to both IPv4 and IPv6 addresses by converting them into a 16‑byte representation. Two generic constructions are defined—one using a 128‑bit block cipher and the other using a 128‑bit tweakable block cipher—along with three concrete instantiations:¶
ipcrypt-deterministic: Deterministic encryption using AES128 (applied as a single‑block operation).¶
ipcrypt-nd: Non‑deterministic encryption using the KIASU‑BC tweakable block cipher with an 8‑byte tweak.¶
ipcrypt-ndx: Non‑deterministic encryption using the AES‑XTS tweakable block cipher with a 16‑byte tweak.¶
Deterministic mode produces a 16‑byte ciphertext (enabling format preservation), while non‑deterministic modes prepend a randomly sampled tweak (which MUST be uniformly random when generated, as specified in [RFC4086]) to produce larger ciphertexts that resist correlation attacks.¶
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/jedisct1/draft-denis-ipcrypt.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 17 October 2025.¶
Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
This document specifies methods for the encryption and obfuscation of IP addresses for both operational use and privacy preservation. The objective is to enable network operators, researchers, and privacy advocates to share or analyze data while protecting sensitive address information.¶
This work addresses concerns raised in [RFC7624] regarding confidentiality in the face of pervasive surveillance. For a detailed discussion of the security properties of these methods, see Section 6.5.¶
The main motivations include:¶
Privacy Protection: Encrypting IP addresses prevents the disclosure of user-specific information when data is logged or measured, as discussed in [RFC6973].¶
Format Preservation: Ensuring that the encrypted output remains a valid IP address allows network devices to process the data without modification. See Section 5.2 for details.¶
Mitigation of Correlation Attacks: Deterministic encryption reveals repeated inputs; non‑deterministic modes use a random tweak to obscure linkability while keeping the underlying input confidential. See Section 6 for implementation details.¶
Privacy-Preserving Analytics: Many common operations like counting unique clients or implementing rate limiting can be performed using encrypted IP addresses without ever accessing the original values. This enables privacy-preserving analytics while maintaining functionality.¶
Third-Party Service Integration: IP addresses are private information that should not be sent in cleartext to potentially untrusted third-party services or cloud providers. Using encrypted IP addresses as keys or identifiers allows integration with external services while protecting user privacy.¶
For implementation examples, see Appendix B.¶
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Throughout this document, the following terms and conventions apply:¶
IP Address: An IPv4 or IPv6 address as defined in [RFC4291].¶
16‑Byte Representation: A fixed-length representation used for both IPv4 (via IPv4‑mapped IPv6) and IPv6 addresses.¶
Tweak: A non‑secret, additional input to a tweakable block cipher that further randomizes the output.¶
Deterministic Encryption: Encryption that always produces the same ciphertext for a given input and key.¶
Non‑Deterministic Encryption: Encryption that produces different ciphertexts for the same input due to the inclusion of a randomly sampled tweak.¶
(Input, Tweak) Collision: A scenario where the same input is encrypted with the same tweak; this reveals that the input was repeated but not the input’s value.¶
This section describes the conversion of IP addresses to and from a 16‑byte representation. This conversion is necessary to operate a 128‑bit cipher on both IPv4 and IPv6 addresses.¶
IPv6 addresses are natively 128 bits and are converted directly using network‑byte order (big‑endian) as specified in [RFC4291].¶
Example:¶
IPv6 Address: 2001:0db8:85a3:0000:0000:8a2e:0370:7334 16-Byte Representation: [20 01 0d b8 85 a3 00 00 00 00 8a 2e 03 70 73 34]¶
IPv4 addresses (32 bits) are mapped using the IPv4‑mapped IPv6 format as specified in [RFC4291]:¶
IPv4 Address: 192.0.2.1 16-Byte Representation: [00 00 00 00 00 00 00 00 00 00 FF FF C0 00 02 01]¶
The conversion algorithm is as follows:¶
Examine the first 12 bytes of the 16-byte representation¶
If they match the IPv4‑mapped prefix (10 bytes of 0x00 followed by 0xFF, 0xFF``):¶
Interpret the last 4 bytes as an IPv4 address in dotted‑decimal notation¶
Otherwise:¶
Interpret the 16 bytes as an IPv6 address in colon‑hexadecimal notation¶
(For additional illustration, see Appendix A)¶
This specification defines two generic cryptographic constructions:¶
Valid options for implementing a tweakable block cipher include, but are not limited to:¶
KIASU-BC (see Appendix C for implementation details)¶
AES-XTS (see Section 6.4.2 for usage)¶
Implementers MUST choose a cipher that meets the required security properties and provides robust resistance against related-tweak and other cryptographic attacks.¶
Deterministic encryption applies a 128‑bit block cipher directly to the 16‑byte representation of an IP address. For implementation details, see Appendix B.¶
Note:
All instantiations documented in this specification (ipcrypt-deterministic, ipcrypt-nd, and ipcrypt-ndx)
are invertible - encrypted IP addresses can be decrypted back to their original values using the same key.
For non-deterministic modes, the tweak must be preserved along with the ciphertext to enable decryption.¶
The ipcrypt-deterministic instantiation employs AES128 in a single‑block operation. Since AES128 is a permutation, every distinct 16‑byte input maps to a unique 16‑byte ciphertext, preserving the IP address format.¶
For test vectors, see Appendix D.1.¶
+---------------------+
| IP Address |
| (IPv4 or IPv6) |
+---------------------+
|
v
+---------------------+
| Convert to 16 Bytes |
+---------------------+
|
v
+---------------------+
| AES128 Encrypt |
| (Single Block) |
+---------------------+
|
v
+---------------------+
| 16-Byte Output |
+---------------------+
|
v
+---------------------+
| Convert to IP Format|
+---------------------+
¶
If the 16‑byte ciphertext begins with an IPv4‑mapped prefix, it MUST be rendered as a dotted‑decimal IPv4 address.¶
Otherwise, it is interpreted as an IPv6 address.¶
Note: To ensure IPv4 format preservation, implementers MUST consider using cycle‑walking, a 32-bit random permutation, or an FPE mode if required.¶
Non‑deterministic encryption leverages a tweakable block cipher together with a random tweak. For implementation details, see Appendix B.¶
The encryption process for non-deterministic modes consists of the following steps:¶
Generate a random tweak using a cryptographically secure random number generator¶
Convert the IP address to its 16-byte representation¶
Encrypt the 16-byte representation using the key and the tweak¶
Concatenate the tweak with the encrypted output to form the final ciphertext¶
The tweak is not considered secret and is included in the ciphertext. This allows the same tweak to be used for decryption.¶
The decryption process consists of the following steps:¶
Split the ciphertext into the tweak and the encrypted IP¶
Decrypt the encrypted IP using the key and the tweak¶
Convert the resulting 16-byte representation back to an IP address¶
Although the tweak is generated uniformly at random (and thus may occasionally collide per birthday bounds), such collisions are benign when they occur with different inputs. An (input, tweak) collision reveals that the same input was encrypted with the same tweak but does not disclose the input’s value.¶
The usage limits discussed below apply per cryptographic key; rotating keys can extend secure usage beyond these bounds.¶
The output of non-deterministic encryption is binary data. For applications that require text representation (e.g., logging, JSON encoding, or text-based protocols), the binary output MUST be encoded. Common encoding options include hexadecimal and Base64.¶
The choice of encoding is application-specific and outside the scope of this specification. However, implementations SHOULD document their chosen encoding method clearly.¶
This document defines two concrete instantiations:¶
ipcrypt-nd: Uses the KIASU‑BC tweakable block cipher with an 8‑byte (64‑bit) tweak.
See [KIASU-BC] for details.¶
ipcrypt-ndx: Uses the AES‑XTS tweakable block cipher with a 16‑byte (128‑bit) tweak.
See [XTS-AES] for background. Since only a single block is encrypted, only the first tweak
needs to be computed, avoiding the need for a full key schedule.¶
In both cases, if a tweak is generated randomly, it MUST be uniformly random. Reusing the same randomly generated tweak on different inputs is acceptable from a confidentiality standpoint.¶
For test vectors, see Appendix D.2 and Appendix D.3.¶
The ipcrypt-nd instantiation uses the KIASU‑BC tweakable block cipher with an 8‑byte (64‑bit) tweak. The output is 24 bytes total, consisting of an 8‑byte tweak concatenated with a 16‑byte ciphertext.¶
Random sampling of an 8‑byte tweak yields an expected collision for a specific tweak value after about 2^(64/2) = 2^32 operations. If an (input, tweak) collision occurs, it indicates that the same input was processed with that tweak without revealing the input’s value.¶
These collision bounds apply per cryptographic key. By rotating keys regularly, secure usage can be extended well beyond these bounds. Ultimately, the effective security is determined by the underlying block cipher’s strength (≈2^128 for AES‑128).¶
The ipcrypt-ndx instantiation uses the AES‑XTS tweakable block cipher with a 16‑byte (128‑bit) tweak. The output is 32 bytes total, consisting of a 16‑byte tweak concatenated with a 16‑byte ciphertext.¶
Since only a single block is encrypted, only the first tweak needs to be computed, avoiding the need for a full key schedule. Independent sampling of a 16‑byte tweak results in an expected collision after about 2^(128/2) = 2^64 operations.¶
As with ipcrypt-nd, an (input, tweak) collision reveals repetition without compromising the input value. These limits are per key, and regular key rotation further extends secure usage. The effective security is governed by the strength of AES‑128 (approximately 2^128 operations).¶
Technical Note: For a single block of AES-XTS, the key is split into two halves (K1, K2). The tweak is first encrypted using AES128 with K2 to produce an encrypted tweak (ET). The IP address is then encrypted as: AES128(IP ⊕ ET, K1) ⊕ ET (where ⊕ denotes the bitwise XOR operation). This construction provides the security properties of XTS while only requiring two AES operations per block.¶
function AES_XTS_encrypt(key, tweak, block):
// Split the key into two halves
K1, K2 = split_key(key)
// Encrypt the tweak with the second half of the key
ET = AES128_encrypt(K2, tweak)
// Encrypt the block: AES128(block ⊕ ET, K1) ⊕ ET
return AES128_encrypt(K1, block ⊕ ET) ⊕ ET
¶
Deterministic (ipcrypt-deterministic):
Produces a 16‑byte output; preserves format but reveals repeated inputs.¶
Non‑Deterministic:¶
ipcrypt-nd (KIASU‑BC): Produces a 24‑byte output using an 8‑byte tweak; (input, tweak) collisions reveal repeated inputs (with the same tweak) but not their values.¶
ipcrypt-ndx (AES‑XTS): Produces a 32‑byte output using a 16‑byte tweak; supports higher secure operation counts per key. Since only a single block is encrypted, it avoids the need for a full key schedule.¶
For a detailed discussion of the security properties of each mode, see:¶
Section 6.4.1 and Section 6.4.2 for non-deterministic mode security considerations¶
A permutation ensures distinct inputs yield distinct outputs. However, repeated inputs result in identical ciphertexts, thereby revealing repetition.¶
This property makes deterministic encryption suitable for applications where format preservation is required, but linkability of repeated inputs is acceptable.¶
The inclusion of a random tweak ensures that encrypting the same input generally produces different outputs. In cases where an (input, tweak) collision occurs, an attacker learns only that the same input was processed with that tweak, not the value of the input itself.¶
Security is determined by the underlying block cipher (≈2^128 for AES‑128) on a per-key basis. Key rotation is recommended to extend secure usage beyond the per-key collision bounds.¶
Implementations MUST ensure that:¶
This document does not require any IANA actions.¶
This appendix provides visual representations of the key operations described in this document. For implementation details, see Appendix B.¶
IPv4: 192.0.2.1
|
v
Octets: C0 00 02 01
|
v
16-Byte Array:
[00 00 00 00 00 00 00 00 00 00 | FF FF | C0 00 02 01]
¶
IP Address
|
v
[Convert to 16 Bytes]
|
v
[AES128 Single-Block Encrypt]
|
v
16-Byte Ciphertext
|
v
[Convert to IP Format]
|
v
Encrypted IP Address
¶
IP Address
|
v
[Convert to 16 Bytes] ---> 16-Byte Representation
|
v
[Generate Random 8-Byte Tweak]
|
v
[KIASU-BC Tweakable Encrypt]
|
v
16-Byte Ciphertext
|
v
[Concatenate Tweak || Ciphertext]
|
v
24-Byte Output (ipcrypt-nd)
¶
IP Address
|
v
[Convert to 16 Bytes] ---> 16-Byte Representation
|
v
[Generate Random 16-Byte Tweak]
|
v
[AES-XTS Tweakable Encrypt]
|
v
16-Byte Ciphertext
|
v
[Concatenate Tweak || Ciphertext]
|
v
32-Byte Output (ipcrypt-ndx)
¶
This appendix provides detailed pseudocode for key operations described in this document. For a visual representation of these operations, see Appendix A.¶
For a diagram of this conversion process, see Appendix A.1.¶
function IPv4To16Bytes(ipv4_address):
// Split the IPv4 address into its octets
parts = ipv4_address.split(".")
if length(parts) != 4:
raise Error("Invalid IPv4 address")
// Create a 16-byte array with the IPv4-mapped prefix
bytes16 = [0x00] * 10 // 10 bytes of 0x00
bytes16.append(0xFF) // 11th byte: 0xFF
bytes16.append(0xFF) // 12th byte: 0xFF
// Append each octet (converted to an 8-bit integer)
for part in parts:
bytes16.append(int(part))
return bytes16
¶
Example: For "192.0.2.1", the function returns¶
[00, 00, 00, 00, 00, 00, 00, 00, 00, 00, FF, FF, C0, 00, 02, 01]¶
function IPv6To16Bytes(ipv6_address):
// Parse the IPv6 address into eight 16-bit words.
words = parseIPv6(ipv6_address) // Expands shorthand notation and returns 8 words
bytes16 = []
for word in words:
high_byte = (word >> 8) & 0xFF
low_byte = word & 0xFF
bytes16.append(high_byte)
bytes16.append(low_byte)
return bytes16
¶
Example: For "2001:0db8:85a3:0000:0000:8a2e:0370:7334", the output is the corresponding 16‑byte sequence.¶
function Bytes16ToIP(bytes16):
if length(bytes16) != 16:
raise Error("Invalid byte array")
// Check for the IPv4-mapped prefix
if bytes16[0:10] == [0x00]*10 and bytes16[10] == 0xFF and bytes16[11] == 0xFF:
ipv4_parts = []
for i from 12 to 15:
ipv4_parts.append(str(bytes16[i]))
ipv4_address = join(ipv4_parts, ".")
return ipv4_address
else:
words = []
for i from 0 to 15 step 2:
word = (bytes16[i] << 8) | bytes16[i+1]
words.append(format(word, "x"))
ipv6_address = join(words, ":")
return ipv6_address
¶
function ipcrypt_deterministic(ip_address, key):
bytes16 = convertTo16Bytes(ip_address)
ciphertext = AES128_encrypt(key, bytes16)
encrypted_ip = Bytes16ToIP(ciphertext)
return encrypted_ip
¶
function ipcrypt_nd_encrypt(ip_address, key):
// Step 1: Generate random tweak
tweak = random_bytes(8) // MUST be uniformly random
// Step 2: Convert IP to 16-byte representation
bytes16 = convertTo16Bytes(ip_address)
// Step 3: Encrypt using key and tweak
ciphertext = KIASU_BC_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
result = concatenate(tweak, ciphertext) // 8 bytes || 16 bytes = 24 bytes total
return result
function ipcrypt_nd_decrypt(ciphertext, key):
// Step 1: Split ciphertext into tweak and encrypted IP
tweak = ciphertext[0:8] // First 8 bytes
encrypted_ip = ciphertext[8:24] // Remaining 16 bytes
// Step 2: Decrypt using key and tweak
bytes16 = KIASU_BC_decrypt(key, tweak, encrypted_ip)
// Step 3: Convert back to IP address
ip_address = Bytes16ToIP(bytes16)
return ip_address
¶
function ipcrypt_ndx_encrypt(ip_address, key):
// Step 1: Generate random tweak
tweak = random_bytes(16) // MUST be uniformly random
// Step 2: Convert IP to 16-byte representation
bytes16 = convertTo16Bytes(ip_address)
// Step 3: Encrypt using key and tweak
// Since only a single block is encrypted, only the first tweak needs to be computed
ciphertext = AES_XTS_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
result = concatenate(tweak, ciphertext) // 16 bytes || 16 bytes = 32 bytes total
return result
function ipcrypt_ndx_decrypt(ciphertext, key):
// Step 1: Split ciphertext into tweak and encrypted IP
tweak = ciphertext[0:16] // First 16 bytes
encrypted_ip = ciphertext[16:32] // Remaining 16 bytes
// Step 2: Decrypt using key and tweak
bytes16 = AES_XTS_decrypt(key, tweak, encrypted_ip)
// Step 3: Convert back to IP address
ip_address = Bytes16ToIP(bytes16)
return ip_address
¶
This appendix provides a detailed guide for implementing the KIASU-BC tweakable block cipher. KIASU-BC is based on AES-128 with modifications to incorporate a tweak. For more information about the security properties of KIASU-BC, see [KIASU-BC].¶
KIASU-BC extends AES-128 by incorporating an 8-byte tweak into each round. The tweak is padded to 16 bytes and XORed with the round key at each round of the cipher. This construction is used in the ipcrypt-nd instantiation.¶
The 8-byte tweak is padded to 16 bytes using the following method:¶
Split the 8-byte tweak into four 2-byte pairs¶
Place each 2-byte pair at the start of each 4-byte group¶
Fill the remaining 2 bytes of each group with zeros¶
Example:¶
8-byte tweak: [T0 T1 T2 T3 T4 T5 T6 T7] 16-byte padded: [T0 T1 00 00 T2 T3 00 00 T4 T5 00 00 T6 T7 00 00]¶
Each round of KIASU-BC consists of the following standard AES operations:¶
The key schedule follows the standard AES-128 key expansion:¶
The following pseudocode illustrates the core operations of KIASU-BC:¶
function pad_tweak(tweak):
// Input: 8-byte tweak
// Output: 16-byte padded tweak
padded = [0] * 16
for i in range(0, 8, 2):
padded[i*2] = tweak[i]
padded[i*2+1] = tweak[i+1]
return padded
function kiasu_bc_encrypt(key, tweak, plaintext):
// Input: 16-byte key, 8-byte tweak, 16-byte plaintext
// Output: 16-byte ciphertext
// Expand key and pad tweak
round_keys = expand_key(key)
padded_tweak = pad_tweak(tweak)
// Initial round
state = plaintext
state = add_round_key(state, round_keys[0] ^ padded_tweak)
// Main rounds
for round in range(1, 10):
state = sub_bytes(state)
state = shift_rows(state)
if round < 9:
state = mix_columns(state)
state = add_round_key(state, round_keys[round] ^ padded_tweak)
// Final round
state = sub_bytes(state)
state = shift_rows(state)
state = add_round_key(state, round_keys[10] ^ padded_tweak)
return state
¶
This appendix provides test vectors for all three variants of ipcrypt. Each test vector includes the key, input IP address, and encrypted output. For non-deterministic variants (ipcrypt-nd and ipcrypt-ndx), the tweak value is also included.¶
# Test vector 1 Key: 0123456789abcdeffedcba9876543210 Input IP: 0.0.0.0 Encrypted IP: bde9:6789:d353:824c:d7c6:f58a:6bd2:26eb # Test vector 2 Key: 1032547698badcfeefcdab8967452301 Input IP: 255.255.255.255 Encrypted IP: aed2:92f6:ea23:58c3:48fd:8b8:74e8:45d8 # Test vector 3 Key: 2b7e151628aed2a6abf7158809cf4f3c Input IP: 192.0.2.1 Encrypted IP: 1dbd:c1b9:fff1:7586:7d0b:67b4:e76e:4777¶
# Test vector 1 Key: 0123456789abcdeffedcba9876543210 Input IP: 0.0.0.0 Tweak: 08e0c289bff23b7c Output: 08e0c289bff23b7cb349aadfe3bcef56221c384c7c217b16 # Test vector 2 Key: 1032547698badcfeefcdab8967452301 Input IP: 192.0.2.1 Tweak: 21bd1834bc088cd2 Output: 21bd1834bc088cd2e5e1fe55f95876e639faae2594a0caad # Test vector 3 Key: 2b7e151628aed2a6abf7158809cf4f3c Input IP: 2001:db8::1 Tweak: b4ecbe30b70898d7 Output: b4ecbe30b70898d7553ac8974d1b4250eafc4b0aa1f80c96¶
# Test vector 1 Key: 0123456789abcdeffedcba98765432101032547698badcfeefcdab8967452301 Input IP: 0.0.0.0 Tweak: 21bd1834bc088cd2b4ecbe30b70898d7 Output: 21bd1834bc088cd2b4ecbe30b70898d782db0d4125fdace61db35b8339f20ee5 # Test vector 2 Key: 1032547698badcfeefcdab89674523010123456789abcdeffedcba9876543210 Input IP: 192.0.2.1 Tweak: 08e0c289bff23b7cb4ecbe30b70898d7 Output: 08e0c289bff23b7cb4ecbe30b70898d7766a533392a69edf1ad0d3ce362ba98a # Test vector 3 Key: 2b7e151628aed2a6abf7158809cf4f3c3c4fcf098815f7aba6d2ae2816157e2b Input IP: 2001:db8::1 Tweak: 21bd1834bc088cd2b4ecbe30b70898d7 Output: 21bd1834bc088cd2b4ecbe30b70898d76089c7e05ae30c2d10ca149870a263e4¶
Note: For non-deterministic variants (ipcrypt-nd and ipcrypt-ndx), the tweak values shown are examples. In practice, tweaks MUST be randomly generated for each encryption operation.¶
Implementations SHOULD verify their correctness against these test vectors before deployment.¶
The author gratefully acknowledges the contributions and insightful comments from members of the IETF independent stream community and the broader cryptographic community that have helped shape this specification.¶