.\" $OpenBSD: ChaCha.3,v 1.3 2022/02/18 10:24:32 jsg Exp $ .\" .\" Copyright (c) 2020 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. .\" .Dd $Mdocdate: February 18 2022 $ .Dt CHACHA 3 .Os .Sh NAME .Nm ChaCha_set_key , .Nm ChaCha_set_iv , .Nm ChaCha , .Nm CRYPTO_chacha_20 , .Nm CRYPTO_hchacha_20 , .Nm CRYPTO_xchacha_20 .Nd ChaCha20 stream cipher .Sh SYNOPSIS .In openssl/chacha.h .Ft void .Fo ChaCha_set_key .Fa "ChaCha_ctx *ctx" .Fa "const unsigned char *key" .Fa "unsigned int keybits" .Fc .Ft void .Fo ChaCha_set_iv .Fa "ChaCha_ctx *ctx" .Fa "const unsigned char *iv" .Fa "const unsigned char *counter" .Fc .Ft void .Fo ChaCha .Fa "ChaCha_ctx *ctx" .Fa "unsigned char *out" .Fa "const unsigned char *in" .Fa "size_t len" .Fc .Ft void .Fo CRYPTO_chacha_20 .Fa "unsigned char *out" .Fa "const unsigned char *in" .Fa "size_t len" .Fa "const unsigned char key[32]" .Fa "const unsigned char iv[8]" .Fa "uint64_t counter" .Fc .Ft void .Fo CRYPTO_hchacha_20 .Fa "unsigned char out[32]" .Fa "const unsigned char key[32]" .Fa "const unsigned char iv[16]" .Fc .Ft void .Fo CRYPTO_xchacha_20 .Fa "unsigned char *out" .Fa "const unsigned char *in" .Fa "size_t len" .Fa "const unsigned char key[32]" .Fa "const unsigned char iv[24]" .Fc .Sh DESCRIPTION These functions provide a low-level implementation of the ChaCha stream cipher with 256 and 128-bit keys. The number of rounds is hardcoded to 20; variants with 8 or 12 rounds are not supported. .Pp Instead of using these functions directly, application programs normally use the more portable .Xr EVP_chacha20 3 high-level interface. .Pp The ChaCha state is contained in the .Vt ChaCha_ctx structure and consists of sixteen 32-bit unsigned integers. .Pp For the recommended value of 256 .Fa keybits , .Fn ChaCha_set_key copies 32 bytes (256 bits) from .Fa key to the middle eight integers of the ChaCha state, using little endian order for each integer. For the alternative value of 128 .Fa keybits , only 16 bytes (128 bits) are copied from .Fa key to the ChaCha state, but they are copied twice, once to the second quarter and once to the third quarter. The first quarter of the ChaCha state is set to four constant integers; these constants differ depending on whether .Fa keybits is 128 or 256. The last quarter of the ChaCha state remains unchanged. .Pp .Fn ChaCha_set_iv copies eight bytes (64 bits) from .Fa counter and eight bytes (64 bits) from .Fa iv to the last quarter of the ChaCha state, the counter to the first two integers and the initialization vector to the last two integers, again in little endian order. If .Fa counter is .Dv NULL , the two respective integers are set to 0 instead. The first three quarters of the ChaCha state remain unchanged. .Pp .Fn ChaCha encrypts .Fa len bytes of data from .Fa in to .Fa out using the .Fa ctx that was previously set up with .Fn ChaCha_set_key and .Fn ChaCha_set_iv . Providing an .Fa out buffer of at least .Fa len bytes is the responsibility of the caller. This function can be called multiple times in a row with varying .Fa len arguments. The .Fa len does not need to be a multiple of 64. .Pp .Fn CRYPTO_chacha_20 encrypts .Fa len bytes of data from .Fa in to .Fa out in a one-shot operation, using the given .Fa key and .Fa iv as described for .Fn ChaCha_set_key and .Fn ChaCha_set_iv and copying the less significant half of .Fa counter to the first counter integer in the initial ChaCha state and the more significant half to the second integer. Providing an .Fa out buffer of at least .Fa len bytes is again the responsibility of the caller. The maximum supported value for .Fa len is 2^32 \- 1. .Pp XChaCha is a variant of ChaCha designed to support longer nonces, just like XSalsa20 is a variant of Salsa20 supporting longer nonces. .Pp .Fn CRYPTO_xchacha_20 encrypts .Fa len bytes of data from .Fa in to .Fa out in a one-shot operation with the XChaCha algorithm, using the given .Fa key and .Fa iv . It is equivalent to .Fn CRYPTO_chacha_20 with the last third of .Fa iv , a .Fa counter of 0, and a key generated with .Fn CRYPTO_hchacha_20 from the first two thirds of .Fa iv . .Sh SEE ALSO .Xr crypto 3 , .Xr EVP_chacha20 3 .Rs .%A Daniel J. Bernstein .%T ChaCha, a variant of Salsa20 .%U https://cr.yp.to/chacha/chacha-20080128.pdf .%C Chicago .%D January 28, 2008 .Re .Rs .%A Daniel J. Bernstein .%T Extending the Salsa20 nonce .%U https://cr.yp.to/snuffle/xsalsa-20110204.pdf .%C Chicago .%D August 22, 2017 .Re .Sh STANDARDS RFC 8439: ChaCha20 and Poly1305 for IETF Protocols .Pp Note that the standard specifies a 32-bit counter and a 96-bit initialization vector whereas this implementation follows Bernstein's original specification and uses a 64-bit counter and a 64-bit initialization vector. .Pp These functions are specific to LibreSSL and not provided by OpenSSL. BoringSSL does provide .Fn CRYPTO_chacha_20 , but with an incompatible interface, taking a 96-bit .Fa iv and a 32-bit .Fa counter . .Sh HISTORY .Fn ChaCha_set_key , .Fn ChaCha_set_iv , .Fn ChaCha , and .Fn CRYPTO_chacha_20 first appeared in .Ox 5.6 . .\" Committed on May 1, 2014. .\" BoringSSL added CRYPTO_chacha_20 on June 20, 2014. .Pp .Fn CRYPTO_hchacha_20 and .Fn CRYPTO_xchacha_20 first appeared in .Ox 6.5 . .Sh AUTHORS .An -nosplit This implementation was written by .An Daniel J. Bernstein Aq Mt djb@cr.yp.to . The API layer was added by .An Joel Sing Aq Mt jsing@openbsd.org for ChaCha, and for XChaCha by .An David Gwynne Aq Mt dlg@openbsd.org .