155 lines
4.3 KiB
Go
155 lines
4.3 KiB
Go
package digest
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import (
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"crypto"
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"fmt"
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"hash"
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"io"
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)
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// Algorithm identifies and implementation of a digester by an identifier.
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// Note the that this defines both the hash algorithm used and the string
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// encoding.
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type Algorithm string
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// supported digest types
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const (
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SHA256 Algorithm = "sha256" // sha256 with hex encoding
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SHA384 Algorithm = "sha384" // sha384 with hex encoding
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SHA512 Algorithm = "sha512" // sha512 with hex encoding
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// Canonical is the primary digest algorithm used with the distribution
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// project. Other digests may be used but this one is the primary storage
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// digest.
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Canonical = SHA256
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)
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var (
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// TODO(stevvooe): Follow the pattern of the standard crypto package for
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// registration of digests. Effectively, we are a registerable set and
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// common symbol access.
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// algorithms maps values to hash.Hash implementations. Other algorithms
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// may be available but they cannot be calculated by the digest package.
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algorithms = map[Algorithm]crypto.Hash{
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SHA256: crypto.SHA256,
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SHA384: crypto.SHA384,
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SHA512: crypto.SHA512,
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}
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)
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// Available returns true if the digest type is available for use. If this
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// returns false, New and Hash will return nil.
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func (a Algorithm) Available() bool {
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h, ok := algorithms[a]
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if !ok {
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return false
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}
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// check availability of the hash, as well
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return h.Available()
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}
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func (a Algorithm) String() string {
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return string(a)
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}
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// Size returns number of bytes returned by the hash.
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func (a Algorithm) Size() int {
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h, ok := algorithms[a]
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if !ok {
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return 0
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}
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return h.Size()
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}
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// Set implemented to allow use of Algorithm as a command line flag.
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func (a *Algorithm) Set(value string) error {
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if value == "" {
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*a = Canonical
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} else {
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// just do a type conversion, support is queried with Available.
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*a = Algorithm(value)
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}
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return nil
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}
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// New returns a new digester for the specified algorithm. If the algorithm
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// does not have a digester implementation, nil will be returned. This can be
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// checked by calling Available before calling New.
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func (a Algorithm) New() Digester {
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return &digester{
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alg: a,
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hash: a.Hash(),
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}
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}
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// Hash returns a new hash as used by the algorithm. If not available, the
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// method will panic. Check Algorithm.Available() before calling.
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func (a Algorithm) Hash() hash.Hash {
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if !a.Available() {
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// NOTE(stevvooe): A missing hash is usually a programming error that
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// must be resolved at compile time. We don't import in the digest
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// package to allow users to choose their hash implementation (such as
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// when using stevvooe/resumable or a hardware accelerated package).
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//
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// Applications that may want to resolve the hash at runtime should
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// call Algorithm.Available before call Algorithm.Hash().
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panic(fmt.Sprintf("%v not available (make sure it is imported)", a))
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}
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return algorithms[a].New()
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}
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// FromReader returns the digest of the reader using the algorithm.
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func (a Algorithm) FromReader(rd io.Reader) (Digest, error) {
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digester := a.New()
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if _, err := io.Copy(digester.Hash(), rd); err != nil {
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return "", err
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}
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return digester.Digest(), nil
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}
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// FromBytes digests the input and returns a Digest.
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func (a Algorithm) FromBytes(p []byte) Digest {
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digester := a.New()
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if _, err := digester.Hash().Write(p); err != nil {
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// Writes to a Hash should never fail. None of the existing
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// hash implementations in the stdlib or hashes vendored
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// here can return errors from Write. Having a panic in this
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// condition instead of having FromBytes return an error value
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// avoids unnecessary error handling paths in all callers.
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panic("write to hash function returned error: " + err.Error())
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}
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return digester.Digest()
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}
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// TODO(stevvooe): Allow resolution of verifiers using the digest type and
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// this registration system.
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// Digester calculates the digest of written data. Writes should go directly
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// to the return value of Hash, while calling Digest will return the current
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// value of the digest.
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type Digester interface {
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Hash() hash.Hash // provides direct access to underlying hash instance.
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Digest() Digest
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}
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// digester provides a simple digester definition that embeds a hasher.
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type digester struct {
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alg Algorithm
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hash hash.Hash
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}
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func (d *digester) Hash() hash.Hash {
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return d.hash
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}
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func (d *digester) Digest() Digest {
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return NewDigest(d.alg, d.hash)
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}
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