44da954565
To make the definition of supported digests more clear, we have refactored the digest package to have a special Algorithm type. This represents the digest's prefix and we associated various supported hash implementations through function calls. Signed-off-by: Stephen J Day <stephen.day@docker.com>
95 lines
2.6 KiB
Go
95 lines
2.6 KiB
Go
package digest
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import (
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"crypto"
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"hash"
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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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TarsumV1SHA256 Algorithm = "tarsum+v1+sha256" // supported tarsum version, verification only
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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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// 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, nil is
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// returned. Make sure to check Available before calling.
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func (a Algorithm) Hash() hash.Hash {
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if !a.Available() {
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return nil
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}
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return algorithms[a].New()
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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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