From b5b943f7b6edcec1f3dcdec0b6dd8bf8f049e3a3 Mon Sep 17 00:00:00 2001 From: Graham Date: Sun, 20 Dec 2020 17:57:49 +0000 Subject: [PATCH] Add documentation summarising the cryptography used by the client Signed-off-by: Graham --- share/doc/cryptography.md | 75 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 share/doc/cryptography.md diff --git a/share/doc/cryptography.md b/share/doc/cryptography.md new file mode 100644 index 00000000..389fdc60 --- /dev/null +++ b/share/doc/cryptography.md @@ -0,0 +1,75 @@ +# Cryptography + +## ISAAC + +[ISAAC][isaac] is a cryptographically-secure pseudorandom number generator. By +combining its output with addition/subtraction, the client uses it as a stream +cipher to encrypt the opcodes of packets. + +It was implemented to break packet injection bots, such as AutoRune. While +using ISAAC in this manner only provides confidentiality, and not authenticity +or integrity, if the opcodes are tampered with the packet lengths will no +longer be in sync between the client and server. This causes one or both of the +endpoints to read garbage opcodes, though the garbage opcodes may happen to +match valid packets for a while. Eventually, one or both endpoints will detect +an invalid packet and close the connection. + +## RSA + +[RSA][rsa] is an asymmetric encryption and signature algorithm. The client uses +it to protect the user's password and the session's symmetric key during the +login process. + +In later revisions, the JS5 master index is signed with Jagex's private key. +This change was made around the time the native libraries were moved into the +cache, ensuring that Jagex's code-signed applet could not be used to run +arbitrary native code if an attacker tampers with the JS5 connection. This +change was probably required by Jagex's certificate authority. + +Jagex used a 512-bit RSA key when build 550 was released, and due to the size +of the output buffer in the client, the maximum key size is 1,008 bits. Both of +these sizes are considered insecure by modern standards, and Jagex's 512-bit +private key was factored in 2016. + +Textbook RSA is used, rather than a secure padding scheme, which leads to +[several weaknesses][textbook-rsa]. + +## SHA-1 + +[SHA-1][sha1] is a cryptographic hash function. It is used to verify the +integrity of the game's code. SHA-1 is no longer secure. + +## XTEA + +[XTEA][xtea] is a symmetric block cipher. It is primarily used to encrypt +location files in the cache, reportedly to prevent bots from performing +path-finding across the entire map - the server only provides keys for a +location file when the player is within or adjacent to it. + +It is used in [Electronic codebook (ECB)][ecb] mode. ECB is theoretically +insecure, however, as the location files are compressed before encryption it is +difficult to make use of this insecurity in practice. + +The location files do not contain padding, and therefore the last 0-7 bytes are +leaked. This has no practical impact as they only contain a portion of the gzip +or bzip2 trailer. + +It is also used (in ECB mode, but with padding) to encrypt the player's email +address in the create account packet, with the symmetric key encrypted with RSA. +XTEA is used as email addresses may sometimes be too long to be encrypted +directly by Jagex's 512-bit RSA key. + +## Whirlpool + +[Whirlpool][whirlpool] is a cryptographic hash function. It is not used in +build 550, however, it is included here for completeness as it is supported by +OpenRS2's cache library. It is used to verify the integrity of native libraries +stored in the cache. + +[ecb]: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_codebook_(ECB) +[isaac]: https://burtleburtle.net/bob/rand/isaacafa.html +[rsa]: https://en.wikipedia.org/wiki/RSA_(cryptosystem) +[sha1]: https://en.wikipedia.org/wiki/SHA-1 +[textbook-rsa]: https://en.wikipedia.org/wiki/RSA_(cryptosystem)#Attacks_against_plain_RSA +[whirlpool]: https://en.wikipedia.org/wiki/Whirlpool_(hash_function) +[xtea]: https://en.wikipedia.org/wiki/XTEA