NAME

       ecm - integer factorization using ECM, P-1 or P+1

SYNOPSIS

       ecm [options] B1 [B2min-B2max | B2]

DESCRIPTION

       ecm is an integer factoring program using the Elliptic Curve Method (ECM), the P-1 method,
       or the P+1 method. The following sections describe parameters relevant to these
       algorithms.

STEP 1 AND STEP 2 BOUND PARAMETERS

       B1
           B1 is the step 1 bound. It is a mandatory parameter. It can be given either in integer
           format (for example 3000000) or in floating-point format (3000000.0 or 3e6). The
           largest possible B1 value is 9007199254740996 for P-1, and ULONG_MAX or
           9007199254740996 (whichever is smaller) for ECM and P+1. All primes 2 <= p <= B1 are
           processed in step 1.

       B2
           B2 is the step 2 bound. It is optional: if omitted, a default value is computed from
           B1, which should be close to optimal. Like B1, it can be given either in integer or in
           floating-point format. The largest possible value of B2 is approximately 9e23, but
           depends on the number of blocks k if you specify the -k option. All primes B1 <= p <=
           B2 are processed in step 2. If B2 < B1, no step 2 is performed.

       B2min-B2max
           alternatively one may use the B2min-B2max form, which means that all primes B2min <= p
           <= B2max should be processed. Thus specifying B2 only corresponds to B1-B2. The values
           of B2min and B2max may be arbitrarily large, but their difference must not exceed
           approximately 9e23, subject to the number of blocks k.

FACTORING METHOD

       -pm1
           Perform P-1 instead of the default method (ECM).

       -pp1
           Perform P+1 instead of the default method (ECM).

GROUP AND INITIAL POINT PARAMETERS

       -x0 x
           [ECM, P-1, P+1] Use x (arbitrary-precision integer or rational) as initial point. For
           example, -x0 1/3 is valid. If not given, x is generated from the sigma value for ECM,
           or at random for P-1 and P+1.

       -sigma s
           [ECM] Use s (arbitrary-precision integer) as curve generator. If omitted, s is
           generated at random.

       -A a
           [ECM] Use a (arbitrary-precision integer) as curve parameter. If omitted, is it
           generated from the sigma value.

       -go val
           [ECM, P-1, P+1] Multiply the initial point by val, which can any valid expression,
           possibly containing the special character N as place holder for the current input
           number. Example:

               ecm -pp1 -go "N^2-1" 1e6 < composite2000

STEP 2 PARAMETERS

       -k k
           [ECM, P-1, P+1] Perform k blocks in step 2. For a given B2 value, increasing k
           decreases the memory usage of step 2, at the expense of more cpu time.

       -treefile file
           Stores some tables of data in disk files to reduce the amount of memory occupied in
           step 2, at the expense of disk I/O. Data will be written to files file.1, file.2 etc.
           Does not work with fast stage 2 for P+1 and P-1.

       -power n
           [ECM, P-1] Use x^n for Brent-Suyama's extension (-power 1 disables Brent-Suyama's
           extension). The default polynomial is chosen depending on the method and B2. For P-1
           and P+1, disables the fast stage 2. For P-1, n must be even.

       -dickson n
           [ECM, P-1] Use degree-n Dickson's polynomial for Brent-Suyama's extension. For P-1 and
           P+1, disables the fast stage 2. Like for -power, n must be even for P-1.

       -maxmem n
           Use at most n megabytes of memory in stage 2.

       -ntt, -no-ntt
           Enable or disable the Number-Theoretic Transform code for polynomial arithmetic in
           stage 2. With NTT, dF is chosen to be a power of 2, and is limited by the number
           suitable primes that fit in a machine word (which is a limitation only on 32 bit
           systems). The -no-ntt variant uses more memory, but is faster than NTT with large
           input numbers. By default, NTT is used for P-1, P+1 and for ECM on numbers of size at
           most 30 machine words.

OUTPUT

       -q
           Quiet mode. Found factorizations are printed on standard output, with factors
           separated by white spaces, one line per input number (if no factor was found, the
           input number is simply copied).

       -v
           Verbose mode. More information is printed, more -v options increase verbosity. With
           one -v, the kind of modular multiplication used, initial x0 value, step 2 parameters
           and progress, and expected curves and time to find factors of different sizes for ECM
           are printed. With -v -v, the A value for ECM and residues at the end of step 1 and
           step 2 are printed. More -v print internal data for debugging.

       -timestamp
           Print a time stamp whenever a new ECM curve or P+1 or P-1 run is processed.

MODULAR ARITHMETIC OPTIONS

       Several algorithms are available for modular multiplication. The program tries to find the
       best one for each input; one can force a given method with the following options.

       -mpzmod
           Use GMP's mpz_mod function (sub-quadratic for large inputs, but induces some overhead
           for small ones).

       -modmuln
           Use Montgomery's multiplication (quadratic version). Usually best method for small
           input.

       -redc
           Use Montgomery's multiplication (sub-quadratic version). Theoretically optimal for
           large input.

       -nobase2
           Disable special base-2 code (which is used when the input number is a large factor of
           2^n+1 or 2^n-1, see -v).

       -base2 n
           Force use of special base-2 code, input number must divide 2^n+1 if n > 0, or 2^|n|-1
           if n < 0.

FILE I/O

       The following options enable one to perform step 1 and step 2 separately, either on
       different machines, at different times, or using different software (in particular, George
       Woltman's Prime95/mprime program can produce step 1 output suitable for resuming with
       GMP-ECM). It can also be useful to split step 2 into several runs, using the B2min-B2max
       option.

       -inp file
           Take input from file file instead of from standard input.

       -save file
           Save result of step 1 in file. If file exists, an error is raised. Example: to perform
           only step 1 with B1=1000000 on the composite number in the file "c155" and save its
           result in file "foo", use

               ecm -save foo 1e6 1 < c155

       -savea file
           Like -save, but appends to existing files.

       -resume file
           Resume residues from file, reads from standard input if file is "-". Example: to
           perform step 2 following the above step 1 computation, use

               ecm -resume foo 1e6

       -chkpoint file
           Periodically write the current residue in stage 1 to file. In case of a power failure,
           etc., the computation can be continued with the -resume option.

               ecm -chkpnt foo -pm1 1e10 < largenumber.txt

LOOP MODE

       The “loop mode” (option -c n) enables one to run several curves on each input number. The
       following options control its behavior.

       -c n
           Perform n runs on each input number (default is one). This option is mainly useful for
           P+1 (for example with n=3) or for ECM, where n could be set to the expected number of
           curves to find a d-digit factor with a given step 1 bound. This option is incompatible
           with -resume, -sigma, -x0. Giving -c 0 produces an infinite loop until a factor is
           found.

       -one
           In loop mode, stop when a factor is found; the default is to continue until the
           cofactor is prime or the specified number of runs are done.

       -b
           Breadth-first processing: in loop mode, run one curve for each input number, then a
           second curve for each one, and so on. This is the default mode with -inp.

       -d
           Depth-first processing: in loop mode, run n curves for the first number, then n curves
           for the second one and so on. This is the default mode with standard input.

       -ve n
           In loop mode, in the second and following runs, output only expressions that have at
           most n characters. Default is -ve 0.

       -i n
           In loop mode, increment B1 by n after each curve.

       -I n
           In loop mode, multiply B1 by a factor depending on n after each curve. Default is one
           which should be optimal on one machine, while -I 10 could be used when trying to
           factor the same number simultaneously on 10 identical machines.

SHELL COMMAND EXECUTION

       These optins allow for executing shell commands to supplement functionality to GMP-ECM.

       -prpcmd cmd
           Execute command cmd to test primality if factors and cofactors instead of GMP-ECM's
           own functions. The number to test is passed via stdin. An exit code of 0 is
           interpreted as “probably prime”, a non-zero exit code as “composite”.

       -faccmd cmd
           Executes command cmd whenever a factor is found by P-1, P+1 or ECM. The input number,
           factor and cofactor are passed via stdin, each on a line. This could be used i.e. to
           mail new factors automatically:

               ecm -faccmd 'mail -s “$HOSTNAME found a factor”
                               me@myaddress.com' 11e6 < cunningham.in

       -idlecmd cmd
           Executes command cmd before each ECM curve, P-1 or P+1 attempt on a number is started.
           If the exit status of cmd is non-zero, GMP-ECM terminates immediately, otherwise it
           continues normally. GMP-ECM is stopped while cmd runs, offering a way for letting
           GMP-ECM sleep for example while the system is otherwise busy.

MISCELLANEOUS

       -n
           Run the program in “nice” mode (below normal priority).

       -nn
           Run the program in “very nice” mode (idle priority).

       -B2scale f
           Multiply the default step 2 bound B2 by the floating-point value f. Example: -B2scale
           0.5 divides the default B2 by 2.

       -stage1time n
           Add n seconds to stage 1 time. This is useful to get correct expected time with -v if
           part of stage 1 was done in another run.

       -cofdec
           Force cofactor output in decimal (even if expressions are used).

       -h, --help
           Display a short description of ecm usage, parameters and command line options.

       -printconfig
           Prints configuration parameters used for the compilation and exits.

INPUT SYNTAX

       The input numbers can have several forms:

       Raw decimal numbers like 123456789.

       Comments can be placed in the file: everything after “//” is ignored, up to the end of
       line.

       Line continuation. If a line ends with a backslash character “\”, it is considered to
       continue on the next line.

       Common arithmetic expressions can be used. Example: 3*5+2^10.

       Factorial: example 53!.

       Multi-factorial: example 15!3 means 15*12*9*6*3.

       Primorial: example 11# means 2*3*5*7*11.

       Reduced primorial: example 17#5 means 5*7*11*13*17.

       Functions: currently, the only available function is Phi(x,n).

EXIT STATUS

       The exit status reflects the result of the last ECM curve or P-1/P+1 attempt the program
       performed. Individual bits signify particular events, specifically:

       Bit 0
           0 if normal program termination, 1 if error occurred

       Bit 1
           0 if no proper factor was found, 1 otherwise

       Bit 2
           0 if factor is composite, 1 if factor is a probable prime

       Bit 3
           0 if cofactor is composite, 1 if cofactor is a probable prime

       Thus, the following exit status values may occur:

       0
           Normal program termination, no factor found

       1
           Error

       2
           Composite factor found, cofactor is composite

       6
           Probable prime factor found, cofactor is composite

       8
           Input number found

       10
           Composite factor found, cofactor is a probable prime

       14
           Probable prime factor found, cofactor is a probable prime

BUGS

       Report bugs on <https://gitlab.inria.fr/zimmerma/ecm/>.

AUTHORS

       Pierrick Gaudry <gaudry at lix dot polytechnique dot fr> contributed efficient assembly
       code for combined mul/redc;

       Jim Fougeron <jfoug at cox dot net> contributed the expression parser and several
       command-line options;

       Laurent Fousse <laurent at komite dot net> contributed the middle product code, the
       autoconf/automake tools, and is the maintainer of the Debian package;

       Alexander Kruppa <(lastname)al@loria.fr> contributed estimates for probability of success
       for ECM, the new P+1 and P-1 stage 2 (with P.-L. Montgomery), new AMD64 asm mulredc code,
       and some other things;

       Dave Newman <david.(lastname)@jesus.ox.ac.uk> contributed the Kronecker-Schoenhage and NTT
       multiplication code;

       Jason S. Papadopoulos contributed a speedup of the NTT code

       Paul Zimmermann <zimmerma at loria dot fr> is the author of the first version of the
       program and chief maintainer of GMP-ECM.

       Note: email addresses have been obscured, the required substitutions should be obvious.