[#1] Define SDK structure

TODO: Вынести маппинг модель -> grpc в отдельный слой

Signed-off-by: Ivan Pchelintsev <i.pchelintsev@yadro.com>

sdk/src/FrostFS.SDK.Cryptography/Base58.cs

@@ -0,0 +1,77 @@
+using System.Numerics;
+using System.Text;
+
+namespace FrostFS.SDK.Cryptography
+{
+    public static class Base58
+    {
+        public const string Alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
+
+        public static byte[] Base58CheckDecode(this string input)
+        {
+            if (input is null) throw new ArgumentNullException(nameof(input));
+            byte[] buffer = Decode(input);
+            if (buffer.Length < 4) throw new FormatException();
+            byte[] checksum = buffer[0..(buffer.Length - 4)].Sha256().Sha256();
+            if (!buffer.AsSpan(^4).SequenceEqual(checksum.AsSpan(..4)))
+                throw new FormatException();
+            var ret = buffer[..^4];
+            Array.Clear(buffer, 0, buffer.Length);
+            return ret;
+        }
+
+        public static string Base58CheckEncode(this ReadOnlySpan<byte> data)
+        {
+            byte[] checksum = data.ToArray().Sha256().Sha256();
+            Span<byte> buffer = stackalloc byte[data.Length + 4];
+            data.CopyTo(buffer);
+            checksum.AsSpan(..4).CopyTo(buffer[data.Length..]);
+            var ret = Encode(buffer);
+            buffer.Clear();
+            return ret;
+        }
+
+        public static byte[] Decode(string input)
+        {
+            // Decode Base58 string to BigInteger 
+            var bi = BigInteger.Zero;
+            for (int i = 0; i < input.Length; i++)
+            {
+                int digit = Alphabet.IndexOf(input[i]);
+                if (digit < 0)
+                    throw new FormatException($"Invalid Base58 character '{input[i]}' at position {i}");
+                bi = bi * Alphabet.Length + digit;
+            }
+
+            // Encode BigInteger to byte[]
+            // Leading zero bytes get encoded as leading `1` characters
+            int leadingZeroCount = input.TakeWhile(c => c == Alphabet[0]).Count();
+            var leadingZeros = new byte[leadingZeroCount];
+            if (bi.IsZero) return leadingZeros;
+            var bytesWithoutLeadingZeros = bi.ToByteArray(isUnsigned: true, isBigEndian: true);
+            return ArrayHelper.Concat(leadingZeros, bytesWithoutLeadingZeros);
+        }
+
+        public static string Encode(ReadOnlySpan<byte> input)
+        {
+            // Decode byte[] to BigInteger
+            BigInteger value = new(input, isUnsigned: true, isBigEndian: true);
+
+            // Encode BigInteger to Base58 string
+            var sb = new StringBuilder();
+
+            while (value > 0)
+            {
+                value = BigInteger.DivRem(value, Alphabet.Length, out var remainder);
+                sb.Insert(0, Alphabet[(int)remainder]);
+            }
+
+            // Append `1` for each leading 0 byte
+            for (int i = 0; i < input.Length && input[i] == 0; i++)
+            {
+                sb.Insert(0, Alphabet[0]);
+            }
+            return sb.ToString();
+        }
+    }
+}