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Poly1305 is a cryptographic message authentication code (MAC) created by Daniel J. Bernstein. It can be used to verify the data integrity and the authenticity of a message. It has been standardized in RFC 7539.


The original proposal, Poly1305-AES, which uses AES block cipher to expand key, computes a 128-bit (16 bytes) authenticator of a variable-length message, using a 128-bit AES key, a 106-bit additional key, r, and a 128-bit nonce. The message is broken into 16-byte chunks which become coefficients of a polynomial in r, evaluated modulo the prime number 2130−5. The name is derived from this and the use of 2130−5 and the Advanced Encryption Standard. In NaCl Poly1305 is used with Salsa20 instead of AES, in TLS and SSH it is used with ChaCha20 keystream.

Google has selected Poly1305 along with Bernstein's ChaCha20 symmetric cipher as a replacement for RC4 in TLS/SSL, which is used for Internet security.[1] Google's initial implementation is securing https (TLS/SSL) traffic between the Chrome browser on Android phones and Google's websites.[2]

Shortly after Google's adoption for TLS, both ChaCha20 and Poly1305 algorithms have also been used for a new chacha20-poly1305@openssh.com cipher in OpenSSH.[3][4] Subsequently, this made it possible for OpenSSH to not depend on OpenSSL through a compile-time option.[5]


The security of Poly1305-AES is very close to the underlying AES block cipher algorithm. As a result, the only way for an attacker to break Poly1305-AES is to break AES.

For instance, assuming that messages are packets up to 1024 bytes; that the attacker sees 264 messages authenticated under a Poly1305-AES key; that the attacker attempts a whopping 275 forgeries; and that the attacker cannot break AES with probability above δ; then, with probability at least 0.999999-δ, all the 275 are rejected.[6]


Poly1305-AES can be computed at high speed in various CPUs: for an n-byte message, no more than 3.1n+780 Athlon cycles are needed,[6] for example. The author has released optimized implementations for Athlon, Pentium Pro/II/III/M, PowerPC, and UltraSPARC, in addition to non-optimized reference implementations in C and C++.


  1. ^ draft-agl-tls-chacha20poly1305-04 - ChaCha20 and Poly1305 based Cipher Suites for TLS4, draft-mavrogiannopoulos-chacha-tls-03 - The ChaCha Stream Cipher for Transport Layer Security
  2. ^ Google Swaps Out Crypto Ciphers in OpenSSL, InfoSecurity, April 24, 2014
  3. ^ Miller, Damien (2013-12-02). "ssh/PROTOCOL.chacha20poly1305". BSD Cross Reference, OpenBSD src/usr.bin/. Retrieved 2014-12-26. 
  4. ^ Murenin, Constantine A. (2013-12-11). Unknown Lamer, ed. "OpenSSH Has a New Cipher — Chacha20-poly1305 — from D.J. Bernstein". Slashdot. Retrieved 2014-12-26. 
  5. ^ Murenin, Constantine A. (2014-04-30). Soulskill, ed. "OpenSSH No Longer Has To Depend On OpenSSL". Slashdot. Retrieved 2014-12-26. 
  6. ^ a b Bernstein, Daniel J. (2005). "The Poly1305-AES Message-Authentication Code". Fast Software Encryption. Lecture Notes in Computer Science 3557. pp. 32–49. doi:10.1007/11502760_3. ISBN 978-3-540-26541-2. 

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