Source: md5.go in package crypto/md5

go:generate go run gen.go -output md5block.go

Package md5 implements the MD5 hash algorithm as defined in RFC 1321. MD5 is cryptographically broken and should not be used for secure applications.

package md5

import (
	"crypto"
	"encoding/binary"
	"errors"
	"hash"
)

func init() {
	crypto.RegisterHash(crypto.MD5, New)
}

The size of an MD5 checksum in bytes.

const Size = 16

The blocksize of MD5 in bytes.

const BlockSize = 64

const (
	init0 = 0x67452301
	init1 = 0xEFCDAB89
	init2 = 0x98BADCFE
	init3 = 0x10325476
)

digest represents the partial evaluation of a checksum.

type digest struct {
	s   [4]uint32
	x   [BlockSize]byte
	nx  int
	len uint64
}

func (d *digest) Reset() {
	d.s[0] = init0
	d.s[1] = init1
	d.s[2] = init2
	d.s[3] = init3
	d.nx = 0
	d.len = 0
}

const (
	magic         = "md5\x01"
	marshaledSize = len(magic) + 4*4 + BlockSize + 8
)

func (d *digest) MarshalBinary() ([]byte, error) {
	b := make([]byte, 0, marshaledSize)
	b = append(b, magic...)
	b = appendUint32(b, d.s[0])
	b = appendUint32(b, d.s[1])
	b = appendUint32(b, d.s[2])
	b = appendUint32(b, d.s[3])
	b = append(b, d.x[:d.nx]...)
	b = b[:len(b)+len(d.x)-d.nx] // already zero
	b = appendUint64(b, d.len)
	return b, nil
}

func (d *digest) UnmarshalBinary(b []byte) error {
	if len(b) < len(magic) || string(b[:len(magic)]) != magic {
		return errors.New("crypto/md5: invalid hash state identifier")
	}
	if len(b) != marshaledSize {
		return errors.New("crypto/md5: invalid hash state size")
	}
	b = b[len(magic):]
	b, d.s[0] = consumeUint32(b)
	b, d.s[1] = consumeUint32(b)
	b, d.s[2] = consumeUint32(b)
	b, d.s[3] = consumeUint32(b)
	b = b[copy(d.x[:], b):]
	b, d.len = consumeUint64(b)
	d.nx = int(d.len % BlockSize)
	return nil
}

func appendUint64(b []byte, x uint64) []byte {
	var a [8]byte
	binary.BigEndian.PutUint64(a[:], x)
	return append(b, a[:]...)
}

func appendUint32(b []byte, x uint32) []byte {
	var a [4]byte
	binary.BigEndian.PutUint32(a[:], x)
	return append(b, a[:]...)
}

func consumeUint64(b []byte) ([]byte, uint64) {
	return b[8:], binary.BigEndian.Uint64(b[0:8])
}

func consumeUint32(b []byte) ([]byte, uint32) {
	return b[4:], binary.BigEndian.Uint32(b[0:4])
}

New returns a new hash.Hash computing the MD5 checksum. The Hash also implements encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to marshal and unmarshal the internal state of the hash.

func New() hash.Hash {
	d := new(digest)
	d.Reset()
	return d
}

func (d *digest) Size() int { return Size }

func (d *digest) BlockSize() int { return BlockSize }

Note that we currently call block or blockGeneric directly (guarded using haveAsm) because this allows escape analysis to see that p and d don't escape.

	nn = len(p)
	d.len += uint64(nn)
	if d.nx > 0 {
		n := copy(d.x[d.nx:], p)
		d.nx += n
		if d.nx == BlockSize {
			if haveAsm {
				block(d, d.x[:])
			} else {
				blockGeneric(d, d.x[:])
			}
			d.nx = 0
		}
		p = p[n:]
	}
	if len(p) >= BlockSize {
		n := len(p) &^ (BlockSize - 1)
		if haveAsm {
			block(d, p[:n])
		} else {
			blockGeneric(d, p[:n])
		}
		p = p[n:]
	}
	if len(p) > 0 {
		d.nx = copy(d.x[:], p)
	}
	return
}

Make a copy of d so that caller can keep writing and summing.

	d0 := *d
	hash := d0.checkSum()
	return append(in, hash[:]...)
}

Append 0x80 to the end of the message and then append zeros until the length is a multiple of 56 bytes. Finally append 8 bytes representing the message length in bits. 1 byte end marker :: 0-63 padding bytes :: 8 byte length

	tmp := [1 + 63 + 8]byte{0x80}
	pad := (55 - d.len) % 64                             // calculate number of padding bytes
	binary.LittleEndian.PutUint64(tmp[1+pad:], d.len<<3) // append length in bits
	d.Write(tmp[:1+pad+8])

The previous write ensures that a whole number of blocks (i.e. a multiple of 64 bytes) have been hashed.

	if d.nx != 0 {
		panic("d.nx != 0")
	}

	var digest [Size]byte
	binary.LittleEndian.PutUint32(digest[0:], d.s[0])
	binary.LittleEndian.PutUint32(digest[4:], d.s[1])
	binary.LittleEndian.PutUint32(digest[8:], d.s[2])
	binary.LittleEndian.PutUint32(digest[12:], d.s[3])
	return digest
}

Sum returns the MD5 checksum of the data.

func Sum(data []byte) [Size]byte {
	var d digest
	d.Reset()
	d.Write(data)
	return d.checkSum()