mirror of https://github.com/docker/cli.git
54 lines
1.7 KiB
Go
54 lines
1.7 KiB
Go
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package identity
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import (
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cryptorand "crypto/rand"
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"fmt"
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"io"
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"math/big"
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)
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var (
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// idReader is used for random id generation. This declaration allows us to
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// replace it for testing.
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idReader = cryptorand.Reader
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)
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// parameters for random identifier generation. We can tweak this when there is
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// time for further analysis.
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const (
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randomIDEntropyBytes = 17
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randomIDBase = 36
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// To ensure that all identifiers are fixed length, we make sure they
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// get padded out or truncated to 25 characters.
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//
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// For academics, f5lxx1zz5pnorynqglhzmsp33 == 2^128 - 1. This value
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// was calculated from floor(log(2^128-1, 36)) + 1.
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//
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// While 128 bits is the largest whole-byte size that fits into 25
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// base-36 characters, we generate an extra byte of entropy to fill
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// in the high bits, which would otherwise be 0. This gives us a more
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// even distribution of the first character.
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//
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// See http://mathworld.wolfram.com/NumberLength.html for more information.
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maxRandomIDLength = 25
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)
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// NewID generates a new identifier for use where random identifiers with low
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// collision probability are required.
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//
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// With the parameters in this package, the generated identifier will provide
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// ~129 bits of entropy encoded with base36. Leading padding is added if the
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// string is less 25 bytes. We do not intend to maintain this interface, so
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// identifiers should be treated opaquely.
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func NewID() string {
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var p [randomIDEntropyBytes]byte
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if _, err := io.ReadFull(idReader, p[:]); err != nil {
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panic(fmt.Errorf("failed to read random bytes: %v", err))
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}
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p[0] |= 0x80 // set high bit to avoid the need for padding
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return (&big.Int{}).SetBytes(p[:]).Text(randomIDBase)[1 : maxRandomIDLength+1]
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}
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