Configuration

Config

One Config type serves both sides. Some fields apply to one side only. Do not change a Config after passing it to a function of the package. A Config may be reused for many connections.

Field	Side	Default	Rules
GetVerifier func(*ClientHelloInfo) (*Verifier, error)	Server	None, required	See Running a server
Username string	Client	None, required	1 to 255 bytes, hashed as given without SASLprep
Password string	Client	Empty	Hashed as given without SASLprep
ServerName string	Client	Empty, no SNI	A trailing dot is removed. An IP address is not sent
MinGroupBits int	Client	2048 when zero	0 to 8192. Groups with a smaller prime are refused
CipherSuites []uint16	Both	AES-256, then AES-128 when nil	The preference order. See Cipher suites
Rand io.Reader	Both	crypto/rand when nil	Source of the random values and the SRP exponents
KeyLogWriter io.Writer	Both	None	Receives the master secrets in NSS key log format. For debugging only

A client Config that breaks these rules fails the handshake before anything is sent. Server and NewListener need a non-nil Config. The server handshake checks only that GetVerifier is set.

ClientHelloInfo

The argument of GetVerifier.

Field or method	Content
Username string	The SRP username, as sent. 1 to 255 bytes, possibly not valid UTF-8
ServerName string	The SNI host name. Empty when the client sent none
CipherSuites []uint16	The cipher suites the client offered
Conn net.Conn	The underlying connection. Reading from it or writing to it breaks the handshake
Context() context.Context	A child of the context passed to HandshakeContext, canceled when the handshake ends

Constructors

Function	Returns
Client(conn net.Conn, config *Config) *Conn	A client connection over conn
Server(conn net.Conn, config *Config) *Conn	A server connection over conn
Listen(network, laddr string, config *Config) (net.Listener, error)	A listener from net.Listen whose connections are *Conn. Fails when GetVerifier is not set
NewListener(inner net.Listener, config *Config) net.Listener	inner with each accepted connection wrapped in Server

Verifiers and groups

Function or method	Purpose
NewVerifier(group *Group, username, password, salt []byte) (*Verifier, error)	Computes a verifier. The username must be 1 to 255 bytes
NewSalt(rand io.Reader) ([]byte, error)	20 random bytes whose first byte is not zero
SimulatedVerifier(seed []byte, group *Group, username []byte) *Verifier	A stable fake verifier for an unknown username
(*Verifier).Validate() error	Checks the group, a salt of 1 to 255 bytes and a value that is not zero and below N
GroupByBits(bits int) (*Group, error)	The RFC 5054 group of that prime size
GroupByParameters(n, g []byte) (*Group, error)	The RFC 5054 group with that prime and generator
(*Group).Bits() int, N() []byte, Generator() []byte	The prime size and copies of the prime and the generator

Verifiers and groups explains how to use them.