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56 lines
1.8 KiB
Plaintext
56 lines
1.8 KiB
Plaintext
/*
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* Copyright (c) 2013-2016 Galois, Inc.
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* Distributed under the terms of the BSD3 license (see LICENSE file)
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*/
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module Base64 where
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import Cryptol::Extras
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type Enc64 n = 4*(((3-(n%3))%3) + n)/3
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base64enc : {n,m,padZ} (4*(padZ + n)/3 == m, fin n, fin m, padZ == (3-(n%3))%3, 2>=padZ)
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=> [n][8] -> [Enc64 n][8]
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base64enc ns = take `{m} (chunks # padEq)
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where
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chunks = take `{(4*n+2)/3} (base64chunks `{(n+2)/3} (ns # padZero))
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padZero = take `{padZ} [zero | _ <- [1..2] : [_][2]]
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padEq = "=="
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alphabet = ['A'..'Z'] # ['a'..'z'] # ['0'..'9'] # "+/"
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base64chunks : {n} (fin n) => [n*3][8] -> [n*4][8]
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base64chunks m = [basify x | x <- ns]
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where
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ns = split (join m) : [_][6]
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basify : [6] -> [8]
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basify x = alphabet @ x
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// Base64 strings are always in 4n byte chunks, the final chunk might be
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// padded. We can not give a proper cryptol implementation that strips the
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// padding because that would require the type, n, to depend on the value
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// (dependant types). Instead we return possibly excess bytes and, separetely,
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// the number of pad bytes (0,1 or 2) as a value.
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base64dec : {n,m} (fin m, 4*n == 3*m, fin n) => [m][8] -> ([n][8],[2])
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base64dec ms = (split (join [debase x | x <- ms]), nrEq)
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where
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nrEq : [2]
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nrEq = sum [ zero # [x == '='] | x <- take `{2} (reverse ms # [zero,zero]) ]
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debase : [8] -> [6]
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debase x = drop `{2} (debase' x)
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debase' : [8] -> [8]
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debase' x =
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if x >= 'A' /\ x <= 'Z'
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then x - 'A'
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else if x >= 'a' /\ x <= 'z'
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then x - 'a' + 26
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else if x >= '0' /\ x <= '9'
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then x - '0' + 52
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else if x == '+'
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then 62
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else if x == '/'
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then 63
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else zero // Pad bytes are decoded as NUL
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