Obviously, I didn't know about the mathn library when I wrote this:

PRIMES = [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,
                73,79,83,89,97,101,103,107,109,113,
                127,131,137,139,149,151,157,163,167,173,
                179,181,191,193,197,199,211,223,227,229,
                233,239,241,251,257,263,269,271,277,281,
                283,293,307,311,313,317,331,337,347,349,
                353,359,367,373,379,383,389,397,401,409,
                419,421,431,433,439,443,449,457,461,463,
                467,479,487,491,499,503,509,521,523,541,
                547,557,563,569,571,577,587,593,599,601,
                607,613,617,619,631,641,643,647,653,659,
                661,673,677,683,691,701,709,719,727,733,
                739,743,751,757,761,769,773,787,797,809,
                811,821,823,827,829,839,853,857,859,863,
                877,881,883,887,907,911,919,929,937,941,
                947,953,967,971,977,983,991,997,1009,1013,
                1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,
                1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,
                1153,1163,1171,1181,1187,1193,1201,1213,1217,1223,
                1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,
                1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,
                1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,
                1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,
                1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,
                1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,
                1663,1667,1669,1693,1697,1699,1709,1721,1723,1733,
                1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,
                1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,
                1901,1907,1913,1931,1933,1949,1951,1973,1979,1987,
                1993,1997,1999,2003,2011,2017,2027,2029,2039,2053,
                2063,2069,2081,2083,2087,2089,2099,2111,2113,2129,
                2131,2137,2141,2143,2153,2161,2179,2203,2207,2213,
                2221,2237,2239,2243,2251,2267,2269,2273,2281,2287,
                2293,2297,2309,2311,2333,2339,2341,2347,2351,2357,
                2371,2377,2381,2383,2389,2393,2399,2411,2417,2423,
                2437,2441,2447,2459,2467,2473,2477,2503,2521,2531,
                2539,2543,2549,2551,2557,2579,2591,2593,2609,2617,
                2621,2633,2647,2657,2659,2663,2671,2677,2683,2687,
                2689,2693,2699,2707,2711,2713,2719,2729,2731,2741,
                2749,2753,2767,2777,2789,2791,2797,2801,2803,2819,
                2833,2837,2843,2851,2857,2861,2879,2887,2897,2903,
                2909,2917,2927,2939,2953,2957,2963,2969,2971,2999,
                3001,3011,3019,3023,3037,3041,3049,3061,3067,3079,
                3083,3089,3109,3119,3121,3137,3163,3167,3169,3181,
                3187,3191,3203,3209,3217,3221,3229,3251,3253,3257,
                3259,3271,3299,3301,3307,3313,3319,3323,3329,3331,
                3343,3347,3359,3361,3371,3373,3389,3391,3407,3413,
                3433,3449,3457,3461,3463,3467,3469,3491,3499,3511,
                3517,3527,3529,3533,3539,3541,3547,3557,3559,3571,
                3581,3583,3593,3607,3613,3617,3623,3631,3637,3643,
                3659,3671,3673,3677,3691,3697,3701,3709,3719,3727,
                3733,3739,3761,3767,3769,3779,3793,3797,3803,3821,
                3823,3833,3847,3851,3853,3863,3877,3881,3889,3907,
                3911,3917,3919,3923,3929,3931,3943,3947,3967,3989,
                4001,4003,4007,4013,4019,4021,4027,4049,4051,4057,
                4073,4079,4091,4093,4099,4111,4127,4129,4133,4139,
                4153,4157,4159,4177,4201,4211,4217,4219,4229,4231,
                4241,4243,4253,4259,4261,4271,4273,4283,4289,4297,
                4327,4337,4339,4349,4357,4363,4373,4391,4397,4409,
                4421,4423,4441,4447,4451,4457,4463,4481,4483,4493,
                4507,4513,4517,4519,4523,4547,4549,4561,4567,4583,
                4591,4597,4603,4621,4637,4639,4643,4649,4651,4657,
                4663,4673,4679,4691,4703,4721,4723,4729,4733,4751,
                4759,4783,4787,4789,4793,4799,4801,4813,4817,4831,
                4861,4871,4877,4889,4903,4909,4919,4931,4933,4937,
                4943,4951,4957,4967,4969,4973,4987,4993,4999,5003,
                5009,5011,5021,5023,5039,5051,5059,5077,5081,5087,
                5099,5101,5107,5113,5119,5147,5153,5167,5171,5179,
                5189,5197,5209,5227,5231,5233,5237,5261,5273,5279,
                5281,5297,5303,5309,5323,5333,5347,5351,5381,5387,
                5393,5399,5407,5413,5417,5419,5431,5437,5441,5443,
                5449,5471,5477,5479,5483,5501,5503,5507,5519,5521,
                5527,5531,5557,5563,5569,5573,5581,5591,5623,5639,
                5641,5647,5651,5653,5657,5659,5669,5683,5689,5693,
                5701,5711,5717,5737,5741,5743,5749,5779,5783,5791,
                5801,5807,5813,5821,5827,5839,5843,5849,5851,5857,
                5861,5867,5869,5879,5881,5897,5903,5923,5927,5939,
                5953,5981,5987,6007,6011,6029,6037,6043,6047,6053,
                6067,6073,6079,6089,6091,6101,6113,6121,6131,6133,
                6143,6151,6163,6173,6197,6199,6203,6211,6217,6221,
                6229,6247,6257,6263,6269,6271,6277,6287,6299,6301,
                6311,6317,6323,6329,6337,6343,6353,6359,6361,6367,
                6373,6379,6389,6397,6421,6427,6449,6451,6469,6473,
                6481,6491,6521,6529,6547,6551,6553,6563,6569,6571,
                6577,6581,6599,6607,6619,6637,6653,6659,6661,6673,
                6679,6689,6691,6701,6703,6709,6719,6733,6737,6761,
                6763,6779,6781,6791,6793,6803,6823,6827,6829,6833,
                6841,6857,6863,6869,6871,6883,6899,6907,6911,6917,
                6947,6949,6959,6961,6967,6971,6977,6983,6991,6997,
                7001,7013,7019,7027,7039,7043,7057,7069,7079,7103,
                7109,7121,7127,7129,7151,7159,7177,7187,7193,7207,
                7211,7213,7219,7229,7237,7243,7247,7253,7283,7297,
                7307,7309,7321,7331,7333,7349,7351,7369,7393,7411,
                7417,7433,7451,7457,7459,7477,7481,7487,7489,7499,
                7507,7517,7523,7529,7537,7541,7547,7549,7559,7561,
                7573,7577,7583,7589,7591,7603,7607,7621,7639,7643,
                7649,7669,7673,7681,7687,7691,7699,7703,7717,7723,
                7727,7741,7753,7757,7759,7789,7793,7817,7823,7829,
                7841,7853,7867,7873,7877,7879,7883,7901,7907,7919]

class Array
  def inject_with_index(sum=self[0])
    each_with_index{|el,idx|sum=yield(sum,el,idx)}
    sum
  end
end

if ARGV[0] == '-e' #encode
  msg = ARGV[1..-1].join(" ")
  encoded_msg = msg.scan(/./).inject_with_index(1){|prod,charstr,idx|
    prod*(PRIMES[idx]**(charstr[0]+1))}
  puts encoded_msg
elsif ARGV[0]=='-d' #decode
  encoded_msg = ARGV[1].to_i
  decoded_msg = ""
  pos = 0
  while encoded_msg > 1
    count = 0
    until encoded_msg % PRIMES[pos] != 0
      encoded_msg /= PRIMES[pos]
      count += 1
    end
    decoded_msg << (count-1).chr
    pos += 1
  end
  puts decoded_msg
end



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