Initial data commit

Task/Substitution-cipher/RISC-V-Assembly/substitution-cipher.asm

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+# gnu assembler syntax
+substitution_cipher: # char* str (a0), uint len (a1), const char lowerkey[26] (a2), const char upperkey[26] (a3)
+        # set up temporary registers t0, t1, t2, t3
+        li t0, 'a
+        li t1, 'z
+        li t2, 'A
+        li t3, 'Z
+        # char tmp (t4)
+        # char* cipher (t5)
+
+        .dcB:                           # begin loop
+        beqz a1, .dcE                   # break condition
+        lb t4, 0(a0)                    # load one character from a0
+        blt t4, t0, .dcU                # lowercase check
+        bgt t4, t1, .dcI
+        addi t4, t4, -'a
+        mv t5, a2
+        j .dcA
+        .dcU:                           # uppercase check
+        blt t4, t2, .dcI
+        bgt t4, t3, .dcI
+        addi t4, t4, -'A
+        mv t5, a3
+        .dcA:                           # convert and save ciphertext character
+        add t5, t5, t4
+        lb t5, 0(t5)
+        sb t5, 0(a0)
+        .dcI:                           # increment registers
+        addi a1, a1, -1
+        addi a0, a0, 1
+        j .dcB
+        .dcE: ret                       # end loop
+
+# You can use the following cipher keys, which correspond to the Atbash cipher,
+# to test the substitution. These keys are self-inverse, which means that
+# applying them twice to a given plaintext yields the original plaintext again.
+latbash:        .ascii "zyxwvutsrqponmlkjihgfedcba"
+uatbash:        .ascii "ZYXWVUTSRQPONMLKJIHGFEDCBA"
+
+# For keys that are non-self-inverse, you will need to keep a separate set of
+# encryption and decryption keys.
+lzebras:        .ascii "zebrascdfghijklmnopqtuvwxy"
+uzebras:        .ascii "ZEBRASCDFGHIJKLMNOPQTUVWXY"
+ldzebras:       .ascii "ecghbijklmnopqrstdfuvwxyza"
+udzebras:       .ascii "ECGHBIJKLMNOPQRSTDFUVWXYZA"